1. Field of the Invention
The present invention relates to lighted control panels for data entry and control used in day and night operation. More particularly, the invention applies to lighted switch panels used around the periphery of information displays used in cockpits and vehicle crewstations.
2. Description of the Related Art
Current lighted switch panels (i.e. “bezels” or “keypanels”) in aircraft using incandescent lighting are unreliable, require high power (>12 Watts), and require fragile, expensive “flex” cables to connect to the backlight driver. LED lighting has been used to reduce power and improve reliability, but it does not adequately match the dimming curve of incandescent lamps, as will be disclosed below in more detail. Luminance also varies among units, which makes brightness difficult to match in the cockpit. The bezel light sensors also require calibration at the unit top level, and recalibration if a bezel is replaced.
Vehicle crewstation lighting typically consists of multiple display and control panels, each with their lighting. This lighting is typically provided within each device by incandescent light bulbs with special filters that limit their infrared emissions so that they do not interfere with the crew's night-vision-goggles. This lighting is generally referred to as “panel lighting”. The crew uses a “rheostat” or variable voltage source to adjust the panel lighting brightness. This voltage is connected to all control and display units. Since all devices use the same kind of lamps, the brightness among units tracks as the “dimming voltage” is changed.
In addition to panel lighting, crewstations use “multifunction displays” (MFDs) to display essential vehicle information. MFDs use large-active-area cathode ray tubes (CRTs) or liquid crystal displays (LCDs) to convey information. These displays generally vary in size from 3″×3″ to 8″×20″. Since LCDs are a transmissive technology and do not emit light, LCD based MFDs are illuminated by a backlight (using fluorescent lamps or LEDs) located behind the display.
MFDs typically have illuminated switch panels around the periphery of the CRT or LCD. These panels may have “fixed function” switches (rotary knobs or pushbuttons) with labels on the switch or panel, or “soft keys” with variable functions. Fixed functions provide rapid single-action access to major, critical function. Soft keys are labeled by software in the MFD that draws a word or symbol adjacent to the soft key to label its function. A soft key may take on many different functions depending on the MFD software. This allows each soft key to replace hundreds of fixed function switches dramatically reducing the cockpit area needed for controls. Systems can provide unlimited growth in functions without adding any new controls.
Each display typically has a brightness control for the display, separate from the instrument panel lighting. This is necessary to allow the crew to balance the large, bright display area with the instrument panel lighting for optimum viewability under the current lighting conditions. The switch panel brightness is controlled by the instrument panel rheostat.
Vehicles must operate under a wide range of lighting environments. Ambient illumination can vary from 0.1 fc to 10,000 fc or more. To accommodate the changes in illumination as the vehicle moves, some MFDs use automatic brightness control (ABC). ABC works by measuring the light falling on the switch panel and increasing the LCD or CRT brightness as the ambient illumination increases. This helps keep the display from “washing out” and losing contrast in bright sunlight.
Some of the problems with current MFDs relate to the switchpanel, as discussed below.
1. Incandescent lamps are unreliable, especially in high-vibration environments. As much as 30% of MFD failures are in the incandescent panel lighting. LED lighting is starting to be used, but LEDs do not follow the same dimming curve as lamps. This gives rise to abrupt changes in brightness as the rheostat is adjusted.
2. Panels with over 30 switches are common, requiring a high speed, multiple wire (10 to 16 wire) interface from the switchpanel to the MFD processor. This is often done using a costly flexible wiring board to control signal impedances and reduce electromagnetic interference.
3. The ambient light sensor in the switchpanel uses photodiodes with highly variable sensitivities. They are installed behind diffusers and at variable mounting angles that add further uncertainty in their sensitivity. As a result, each switchpanel has different characteristics. To assure all MFDs track, a calibration process is necessary at the next level of assembly, typically the Display Head level. This means that switchpanels are not interchangeable. To replace a switchpanel the unit has to be recalibrated. This makes field replacement of switchpanels impossible, and increases the cost of ownership.
As will be disclosed below, the invention described herein solves all three problems. It controls reliable LED lighting to make it compatible with other incandescent-lighted units. It eliminates the expensive flex board by using a low-cost controller and serial interface. The light sensor is provided with calibration in the switchpanel using non-volatile memory so all panels have the same characteristics. This ambient light value is sent over the same serial interface used to send key information to simplify the MFD and reduce cost.